Anthropogenic emissions, clouds and climate
Clouds are the most important factor controlling the albedo (reflectivity) and hence the temperature of our planet. In fact, the surface of the Earth is covered by clouds 60% of the time on average, and clouds reflect the incoming solar radiation and cause cooling at the surface. Man-made aerosols have a strong influence on cloud optical properties, thus strongly affecting the albedo of the Earth. This climatic effect, called the "indirect aerosol effect" is still the major uncertainty in global climate models.
A recent research project carried out jointly by the Institute for Atmospheric Sciences and Climate (ISAC), the University of Washington, the California Institute of Technology and the University of Helsinki has shown that cloud optical properties depend not only on physical parameters as the size distribution of the aerosol population on which cloud droplets form and grow, as the classical cloud formation theory predicts. Other "chemical factors", due to the anthropogenic emission of gases and particles play an important role in this process. The results of the project have been published in the form of a general discussion in Science, while a more detailed description was later published in Geophysical Research Letters.
These results derive from a collaborative effort of ISAC and the other Institutes mentioned above with the aim of studying the effect of clouds on climate and the influence of anthropogenic emissions on cloud structure and optical properties. The originality and importance of this research consists in the inclusion of "chemical factors" connected to anthropogenic emissions in a mathematical model which describes the formation and evolution of clouds over areas characterised by different levels of emissions: urban areas, rural areas, remote marine locations. Model calculations have shown that some aerosol types containing carbonaceous material favour formation of clouds with an high reflectivity which cause a larger cooling effect on climate with respect to results presently available from global climatic models.
The results of this research project are currently being implemented in large scale climatic models and the international collaborations within this activity ensure continuation of the research over the next years, mainly through funding from the European Commission.
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